Hazardous drinking and alcohol-related problems such as drinking and driving remain prevalent among adolescents and young adults in the U.S. A number of promising environmental strategies have been identified to address these problems, including party dispersal operations, underage decoy operations at retail establishments, and penalties for underage possession and consumption of alcoholic beverages and drinking and driving. Although research indicates that some alcohol control policies (e.g., raising minimum legal drinking age to 21, lowering legal blood alcohol limit for driving) have beneficial effects on alcohol misuse and related problems among young people, much less is known about the effectiveness of local enforcement activities. Research on the effectiveness of local alcohol policy enforcement is limited by substantial variability in the implementation and intensity of enforcement activities, and by methodological challenges in measuring their intensity. The proposed five-year study will advance our understanding of how effective local enforcement strategies are in reducing hazardous drinking and alcohol-related problems such as drinking and driving by young people. The study will capitalize on the Strategic Planning Framework/State Incentive Grant (SPF/SIG) in California from the Center for Substance Abuse Prevention, and will involve 24 cities that are part of the 50-city sample meeting criteria for the Environmental Approaches to Prevention center grant. Half of the cities will be randomly assigned to implement environmental interventions such as party patrol operations, underage decoy operations, and DUI sobriety checkpoints. Archival data will be used to examine intervention effects on alcohol problems among young adults and teens in the 24 cities from 2012 to 2016, such as alcohol-related motor vehicle crashes, violent assaults, injuries, and underage drinking estimates from the California Healthy Kids Survey. Pre- and post-intervention interviews also will be conducted with 2,400 18- to- 30-year-olds to examine intervention effects on possible intervening mechanisms through which local enforcement strategies may affect alcohol use and related problems, including perceived enforcement of alcohol policies, perceived ease of obtaining alcohol, use of social and commercial alcohol sources, and drinking locations. Study aims are: (1) to determine whether there is a significant reduction in community alcohol problems (alcohol-related motor vehicle crashes, violent assaults, injuries, underage drinking), particularly among young people in SPF/SIG intervention cities relative to controls; and (2) to investigate the intervening mechanisms through which local enforcement strategies may affect alcohol use and related problems among young people (e.g., perceived enforcement of alcohol laws).